One of the first things Harmonix is improving in Dance Central 2 is the ability for the Kinect to track two players at the same time with support for drop-in / drop-out gameplay. Once a player leaves the dance floor, the game will focus on the remaining dancer. This is a pretty big deal if you've ever wanted to play Dance Central with a group of your friends, but dread having to wait your turn while everyone goes one at a time. In theory, others can dance along, but only one person can be tracked at a time in Dance Central, so this is a much needed update.
In true Harmonix fashion, Dance Central 2 will also allow for players to import songs from Dance Central, both on the disc and DLC, to Dance Central 2. This is another great move by Harmonix as if you're a dedicated Dance Central fan who has been downloading DLC, the last thing you want to do is lose all of those tracks just because a new game was released. It also helps to dance to some classic songs from the first game every so often.
Dance Central 2 will ship with 40 new tracks, three of which were revealed from the E3 trailer: "This is how we do it" by Montell Jordan, "Bulletproof" by La Roux, and "Like a G6" by Far East Movement. No release date has been announced as of this time, but I'm confident we can see Dance Central 2 this holiday season.